This invention relates to plunger operated liquid dispensers, such as manual pipettors, which are used to dispense liquids. Specifically the invention relates to reliable removal of a liquid from the liquid dispenser. Especially preferable the invention relates to liquid dispensers that are used to handle relatively small sample amounts, such as around one microliter. More specifically the object of the invention is further described in the preambles of the independent claims.
Plunger operated liquid dispensers normally have a function which is called a blow-out. In a blow-out, the distance of the movement of the plunger while removing a liquid (secondary movement) is longer than the movement which has been made by the plunger while receiving the liquid. Thus removal of the liquid can be improved when compared to the situation when the plunger would move the same distance in both the receiving and removal phases. In manual pipettors this function is usually accomplished by two springs, whereas in electronic pipettors this function is possible to accomplish simply by continuing the movement of the plunger with the help of a motor. In both approaches only one plunger is used.
The aforementioned removing problem is greater with small volumes when the diameter of the plunger is small in relation to the removal opening of the tip. What usually happens is that the liquid is not entirely removed from the tip and instead revolves to the outer surface of the tip. Then the liquid drop must be mechanically removed by the user by transferring the liquid mechanically to a wall of the tube which is receiving the sample. Usually the removal pipetting has to be repeated several times. Another solution is to saturate the sample on the tip to a liquid already existing in the tube. In the end, how well the liquid actually transfers and how much still remains in the tip of the pipettor always depends on the carefulness of the person performing the pipetting. Naturally these different kinds of contacts also result in an increase in different contamination risks.
More generally the weakness in pipettors can be said to be this difficult pipetting of small portion volumes. Sample volumes below 10 μL do not come off from the pipettor tip, such that they must be pipetted saturated in a liquid or they must be able to catch on the wall of a test tube. This has resulted in that manual pipettors are used when small volumes are pipetted because manual pipettors enable several removals successively. However, the accuracy is then quite poor.
The international standards allow large CV percentages for 0.1-2.0 μL volumes. However, smaller volumes usually are portion volumes of a test sample which determine the accuracy of the whole test. These standards are clearly drafted taking into consideration the problem caused by the small plunger diameter.
Automatic dispensers which require that a different tip must be used for each sample cannot dispense small volumes, which results in that a lot of reagences must be used. In addition if attempts are made to catch the sample on the bottom of a test tube/plate with these automatic dispensers, the accuracy of the structure of this kind of device must be controlled extremely well, and in the worst case, the situation is such that a bad accuracy appears occasionally.
In turn, in hand held pipettors, especially in 10 μL pipettors, because of different attachment of the tips, catching the sample on a test plate is extremely difficult.
One approach is to use two nested plungers. The inner plunger is a portioning plunger whose movement distance defines the volume to be dispensed. The distance of the movement of this portioning plunger is the same while receiving the liquid and while removing it. The outer plunger is a blow-out plunger, which starts its movement only after the portioning plunger has carried out its removal movement.
Another approach is to use two plungers so that these two plungers act simultaneously during the removal, whereby more pressure is developed to a tip of the pipettor. However, even the smallest drops which tend the remain in the tip are desired to be removed from the tip.